Dumping the load of a truck preferably occurs as a gradual evacuation. With certain materials, however, such as the materials collected from the Canadian oil sands, the contents of the bed can adhere together, and dump as a single unit, or a small number of relatively large units. This phenomenon is referred to as loafing.
The dumping of a load is accomplished by way of a plurality of actuators. In viewing the structure of an actuator, a rod extends from the one side of the piston and outward from the cylinder. When dumping, the actuators extend, that is, hydraulic fluid is evacuated from the rod chamber of the actuator and hydraulic fluid is moved to the head chamber. As a bed is moved to start the dumping motion, the force of the load acts to compress the actuators. As the load continues to shift toward the dumping end of the bed, however, a situation occurs that is commonly referred to as an overrunning load if the load does not proceed gradually to dump from the bed. That is, if the load acts as a loaf, the force of the shifting load causes a moment that exerts a force on the actuator in the extending direction of the actuator.
If the flow of fluid to the head chamber is inadequate to meet the demands of the forcibly extending actuator, an undesirable severe voiding results in the head end of the actuator. In other words, a vacuum develops in the head chamber as the volume of the head chamber extends beyond the volume of the hydraulic fluid flowing to the head chamber. As a result, when the load drops from the bed as a loaf, the vacuum formed in the head chamber causes the actuator to rapidly retract. This significant and undesirable dynamic event can result in discomfort, and even injury to a machine operator, or damage to the machine.
In prior art arrangements, a hydraulic tank is provided as an external source of make-up flow to the hoist valve of the actuator. Unfortunately, however, this passive arrangement is often inadequate to meet the needs of an overrunning actuator, and an alternative solution is desirable.